Snubber circuits are known in the art. Snubber circuits generally are used with switching devices such as power semiconductors. The snubber circuit is designed to provide efficient and reliable operation in a switching circuit by performing one or more different snubbering functions. For example, the snubber circuit suppresses the voltage spikes that are generated across a switch as a result of the switching off of the electrical current as in a switching inverter or chopper circuit having an inductive filter circuit at the output thereof. In particular, the snubber circuit prevents the development of excessive transient voltage peaks across the switching device upon the opening of the switching device and the resultant interruption of current flow through the inductive element of the output filter. The snubber circuit thereby prevents damage to the switching device which otherwise may occur when the transient voltage peaks exceed the rated voltage limits of the switching device.
Snubber circuits also are useful to tailor switching trajectory, reduce switching losses, control the effects of parasitic elements in the circuit, etc. A description of conventional snubber circuits can be found in the following articles: Evans, et al., Analysis of Conventional Snubber Circuits for PWM Inverters Using Bipolar Transistors, IEE Proc., Vol. 135, Pt. B, No. 4, pp. 180-192 (July 1988); McMurray, Optimum Snubbers for Power Semiconductors, IEEE Trans. on Ind. App. (Sept./Oct. 1972); and Steyn, Optimum Size of Dissipative Nonlinear Turn-off, IEE Proc., Vol. 135, Pt. B, No. 4, pp. 165-171 (July 1988). The entire disclosures of the above articles are incorporated herein by reference.
Typically, switch snubbering in a switching inverter or chopper circuit is accomplished using a single resistor-capacitor (R-C) or R-C-Diode (R-C-D) type snubber circuit per switch. Therefore, in a three-phase full bridge inverter with six power switches, there typically are six snubber circuits, or one snubber circuit per switch. Each snubber circuit, however, is required to perform several functions simultaneously. For example, a conventional snubber circuit in an inverter circuit first must limit the peak voltage developed across the power switch upon the interruption of the current flow. In addition, the same snubber circuit must suppress or control any oscillations developed in the inverter circuitry upon the opening of the power switch, particularly in the case where the inverter output circuitry includes inductive components.
There have been several drawbacks, however, associated with conventional snubber circuits. For instance, if the snubber circuit were to provide the best possible circuit performance with respect to each of its intended functions (e.g., to limit peak voltages and to suppress oscillations), the snubber circuit would require different R-C or R-C-D component values. As a particular example, the optimum value of the resistor R required to control the inductive components in an inverter, and therefore suppress oscillations, is typically lower than the optimum value of the resistor R for suppressing the peak voltage developed across the power switch. Moreover, the same type of statements can be made regarding the capacitor C and diode D components in the snubber circuit.
As a result, it has been necessary in the past to accept a trade off as to the different component values in the snubber circuit in view of its intended functions. Snubber optimization has therefore been basically the determination of the best compromise values for the R-C or R-C-D snubber circuit components. Because such a compromise is required, however, it is unlikely that the snubber circuit will be optimized to perform even one of the desired snubbering functions in particular. As a result of such compromise, there have been high losses associated with conventional snubber circuits. These losses have resulted in larger and more expensive components in the snubber circuit, and lower circuit efficiency. Such losses often result in substantial cooling requirements for the inverter circuit, chopper circuit, etc., and in reduced circuit reliability.
In view of the above-mentioned shortcomings associated with existing snubber circuits, there is a strong need in the art for a snubber circuit which can provide improved and/or optimized snubbering for each respective snubbering function. More particularly, there is a strong need for a snubber circuit which provides not only optimal voltage spike suppression, but which also provides optimal oscillation suppression or control, particularly in an inverter circuit.
Furthermore, there is a strong need in the art for a snubber circuit which reduces the loss associated with a switching circuit, thereby resulting in higher circuit efficiency. Moreover, there is a strong need in the art for a snubber circuit which utilizes smaller, less expensive components as compared to existing snubber circuits.